Anaerobic Bioremediation | In Situ Delivery (ISD™) System

Our In Situ Delivery (ISD™) equipment and anaerobic bioremediation products address the critical governing principles that control the success or failure of any in situ anaerobic bioremediation effort. These principles are discussed below in greater detail.

Mass Balance

All remediation processes come down to mass balance. How many pounds or kilograms of a contaminant exists in the subsurface, and how much of this mass can be removed by the proposed remediation system? With reductive dechlorination, the mass balance is a function of the stoichiometry of the oxidation-reduction reactions that govern the biological utilization of a particular compound. For example, oxidation-reduction reactions for the reductive dechlorination of PCE are as follows:

 

Oxidation Reaction: C6H12O6 + 6 H2O → 6 CO2 + 24 H+ + 24 e-

Reduction Reaction: 3 C2Cl4 + 12 H+ + 24 e- → 3 C2H4 + 12 Cl-

Overall Reaction: C6H12O6 + 3 C2Cl4 + 6 H2O → 3 C2H4 + 12 Cl- + 12 H+ + 6 CO2

The C6H12O6 requirement for this reaction is that 1 mole of C6H12O6 can degrade 3 moles of C2Cl4. Converting to the appropriate mass ratio results in the following:

1 unit C6H12O6 : 3 x (180/166) units of C2Cl4, or 1 unit C6H12O6 can degrade more than 3 units of C2Cl4.

Theoretically, anaerobic bioremediation using 1 pound of carbon substrate can degrade 3 pounds of PCE in the subsurface. However, our practical experience has proven that the amount of carbon substrate necessary for complete dechlorination of PCE is a significantly greater mass. Our soluble, nutrient-amended electron donor substrate, Carbstrate™, fully addresses the problems with supplying a large mass of substrate on a site-wide basis.

Hydraulic Influence

ISDconceptual

By utilizing our ISD™ equipment in combination with localized injection and extraction wells, artificial groundwater gradients can be produced within the plume area to induce circulation of a soluble carbon substrate and essential nutrients through the groundwater and smear-zone soil. By placing injection and extraction wells at specific locations, individual “circulation cells” can be created throughout a plume zone (i.e. source area, sidegradient, downgradient, etc.), resulting in:

  • Optimized electron-donor substrate delivery
  • Hydraulic plume control
  • Induced groundwater gradients via GW mounding/drawdown

The importance of hydraulic influence as part of a groundwater remediation process cannot be overstated…groundwater management results in long-term flexibility, accelerated cleanup, and shorter remediation timeframes.

Consistent groundwater recirculation is also the best available technology for approximating complete-mix CSTR conditions in the subsurface. This is an important factor for reductive dechlorination, since research has shown that stable concentrations of hydrogen in groundwater favor dehalogenating bacteria (as opposed to methanogens and acetogens).

Dissolution & Desorption

CSsoilmovement

Contaminants in the subsurface partition into various phases depending upon their specific physical/chemical characteristics. However, contaminants in the adsorbed phase (i.e. the constituents that are bound to the organic soil fraction) usually represents over 70% of the total subsurface mass. Successful in situ reductive dechlorination must address the adsorbed-phase mass in conjunction with the dissolved-phase constituents to achieve site closure. By constantly replacing and re-circulating groundwater within the soil matrix, the our reductive dechlorination process accelerates dissolution of sorbed constituents via:

  • Maximized and continuous smear-zone soil contact
  • Enhanced delivery of biological products
  • Stimulation of a large microbial population

By forcing sorbed constituents into solution, they are rendered bio-available and susceptible to subsequent biological degradation. More importantly, desorption can be enhanced 300% to 600% by stimulating a highly-active microbial community! The combination of these dissolution processes ensures treatment of both sorbed- and dissolved-phase contaminant mass.

Complete Dechlorination without Stall

In the past, many reductive dechlorination projects experienced a “stall” effect that resulted in a buildup of vinyl chloride (VC). Currently, there is significant debate in the industry regarding the need for bioaugmentation (addition of dechlorinating bacterial species such as Dehalococcoides) in order to ensure complete dechlorination. To date, ETEC has performed over 12 full-scale in situ dechlorination projects, and all these projects have shown complete dechlorination to ethene. Like some industry professionals, we believe that complete dechlorination is achieved by providing and maintaining proper subsurface conditions. We believe our success results from the following:

  • Supply of an easily degradable carbon substrate
  • Consistent delivery at controlled concentrations throughout the solvent plume area
  • Availability of specific nutrients (N, P, K, micro-nutrients) to stimulate growth of indigenous dechlorinating bacteria

By providing these components, our reductive dechlorination process has resulted in complete dechlorination and no build-up of hazardous daughter products like VC. An added benefit of our process is the production of significant biomass within the saturated soil matrix. Research and our own project experience has shown that the slow decomposition of this accumulated biomass serves to continue the dechlorination process, reducing or eliminating contaminant rebound following active treatment.